1. Field of the Invention
The present invention relates to a light-emitting element which sandwiches a thin film containing a light-emitting material between electrodes and which emits light by applying current. Moreover, the present invention relates to a display device and an electronic appliance which use the light-emitting element.
2. Related Art
A display using a thin film light-emitting element of a self-light-emitting type, which emits light by itself when current is applied, has been extensively developed.
This thin film light-emitting element emits light by connecting an electrode to a single-layer or multilayer thin film formed using one or both of organic compound and inorganic compound and by applying current. Such a thin film light-emitting element is expected to reduce the power consumption, occupy smaller space, and increase the visibility, and the market is also expected to expand further.
It has become possible to manufacture an element which emits light more efficiently than before by dividing the function for each layer of a light-emitting element having a multilayer structure (for example, see Reference 1: Applied Physics Letters, Vol. 51, No. 12, 913-915 (1987) by C. W. Tang et al.).
A thin film light-emitting element having a multilayer structure has a light-emitting laminated body sandwiched between an anode and a cathode. The light-emitting laminated body comprises a hole-injecting layer, a hole-transporting layer, a light-emitting layer, an electron-transporting layer, an electron-injecting layer, and the like. Among these layers, the hole-injecting layer, the hole-transporting layer, the electron-transporting layer, and the electron-injecting layer are not always employed depending on the element structure.
The hole-injecting layer in the light-emitting laminated body as above is formed by selecting a material which can inject holes comparatively easily from a metal electrode into a layer mainly containing organic compound. The electron-transporting layer in the light-emitting laminated body is formed by selecting a material being superior in electron-transporting properties. Thus, each layer in the light-emitting laminated body is formed by selecting a material superior in each function.
However, a material which can inject electrons comparatively easily from an electrode into a material mainly containing organic compound, or a material mainly containing organic compound which can transport electrons at a predetermined mobility or more are very limited. As is clear from the limitation on the material, the injection of the electrons from the electrode into the layer mainly containing the organic compound is originally rare to occur. This causes the problem that the drive voltage increases drastically over time.